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Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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Priority-Based Flow Control

In a data center network, priority-based flow control (PFC) manages large bursts of one traffic type in multiprotocol links so that it does not affect other traffic types and no frames are lost due to congestion.

When PFC detects congestion on a queue for a specified priority, it sends a pause frame for the 802.1p priority traffic to the transmitting device. In this way, PFC ensures that large amounts of queued LAN traffic do not cause storage traffic to be dropped, and that storage traffic does not result in high latency for high-performance computing (HPC) traffic between servers.

PFC enhances the existing 802.3x pause and 802.1p priority capabilities to enable flow control based on 802.1p priorities (classes of service). Instead of stopping all traffic on a link (as performed by the traditional Ethernet pause mechanism), PFC pauses traffic on a link according to the 802.1p priority set on a traffic type. You can create lossless flows for storage and server traffic while allowing for loss in case of LAN traffic congestion on the same physical interface.

The following illustration shows how PFC handles traffic congestion by pausing the transmission of incoming traffic with dot1p priority 3.

Figure 1. Priority-Based Flow Control Illustration of the priority-based flow control.
In the system, PFC is implemented as follows:
  • PFC is supported on specified 802.1p priority traffic (dot1p 0 to 7) and is configured per interface. However, only two lossless queues are supported on an interface: one for Fibre Channel over Ethernet (FCoE) converged traffic and one for Internet Small Computer System Interface (iSCSI) storage traffic. Configure the same lossless queues on all ports.
  • A dynamic threshold handles intermittent traffic bursts and varies based on the number of PFC priorities contending for buffers, while a static threshold places an upper limit on the transmit time of a queue after receiving a message to pause a specified priority. PFC traffic is paused only after surpassing both static and dynamic thresholds for the priority specified for the port.
  • By default, PFC is enabled when you enabled DCB. When you enable DCB globally, you cannot simultaneously enable TX and RX on the interface for flow control and link-level flow control is disabled.
  • Buffer space is allocated and de-allocated only when you configure a PFC priority on the port.
  • PFC delay constraints place an upper limit on the transmit time of a queue after receiving a message to pause a specified priority.
  • By default, PFC is enabled on an interface with no dot1p priorities configured. You can configure the PFC priorities if the switch negotiates with a remote peer using DCBX. During DCBX negotiation with a remote peer:
    • DCBx communicates with the remote peer by link layer discovery protocol (LLDP) type, length, value (TLV) to determine current policies, such as PFC support and enhanced transmission selection (ETS) BW allocation.
    • If the negotiation succeeds and the port is in DCBX Willing mode to receive a peer configuration, PFC parameters from the peer are used to configured PFC priorities on the port. If you enable the link-level flow control mechanism on the interface, DCBX negotiation with a peer is not performed.
    • If the negotiation fails and PFC is enabled on the port, any user-configured PFC input policies are applied. If no PFC dcb-map has been previously applied, the PFC default setting is used (no priorities configured). If you do not enable PFC on an interface, you can enable the 802.3x link-level pause function. By default, the link-level pause is disabled, when you disable DCBx and PFC. If no PFC dcb-map has been applied on the interface, the default PFC settings are used.
  • PFC supports buffering to receive data that continues to arrive on an interface while the remote system reacts to the PFC operation.
  • PFC uses the DCB MIB IEEE802.1azd2.5 and the PFC MIB IEEE802.1bb-d2.2.
If DCBx negotiation is not successful (for example, due to a version or TLV mismatch), DCBx is disabled and you cannot enable PFC or ETS.

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